Infrared spectroscopy is a really useful analytical technique for determining whether there are key bonds in a molecule (C=O, O-H, N-H, C-O, C=C) and of course once we have this information, we can deduce which functional groups are present.
All chemical bonds vibrate.
We can think of a chemical bond between two atoms as a spring. The ‘vibration’ of this bond is simply the spring stretching and compressing about its equilibrium position.
Compressing or stretching the spring requires energy and as we compress / stretch the spring, a restoring force pulls it back to its equilibrium length.
The frequency of this vibration depends on the mass of the atoms (heavier atoms cause the bond / spring to vibrate more strongly) and the bond strength (a stiff spring equates to a strong bond with more electron density between the atoms e.g. C=O and vibrates more quickly).
A bond will vibrate more strongly if it absorbs energy. The frequency of the energy needed depends on the bond in question but it will lie in the infrared region of the electromagnetic spectrum (ð = 1014-1013Hz; ð = 2.5-15Ξm).
The potential energy curve for a simple diatomic molecule (purple line) is shown below. It is a slightly different shape to that of a model spring as the bond becomes weaker was we stretch it and eventually the bond breaks if we stretch it far enough.
In molecules the vibrational energy levels are quantised and not evenly spaced so to move from v=0 to v=1 (making the bond vibrate more strongly) requires a specific frequency of IR energy to be absorbed. The frequency of energy needed to make an O-H bond vibrate more strongly will not be the same as for a C=O bond.
We can clearly see this on an IR spectrum. If we shine all the frequencies of IR light on a sample of ethanol, specific bonds will absorb specific frequencies and we get a peak (actually a trough!) on the IR spectrum that is called an absorption band.
We can match individual peaks to specific bonds using a data sheet.
The units for the x-axis are in wavenumber, cm-1, which is essentially a made-up unit … the idea is that
wavenumber = 1/ð
and since
c = ð ð (c= speed of light, ð = wavelength and ð = frequency)
then 1/ð is a direct measure of frequency but with easier numbers. Note that the x-axis is non-linear so take care reading the wavenumber for a peak off a spectrum.
There must be a change in the dipole moment of a molecule during the vibration for the vibration to show up on the spectrum, which means that homonuclear diatomic molecules such as H2 and Cl2 will not produce an IR spectrum.
dipole moment = charge x distance
More complex molecules have many different vibrational modes …
Water has a symmetric stretch which shows up on the IR spectrum as a broad peak at 3652cm-1
Original animation by Nick Greaves for ChemTube3D
Water has a asymmetric stretch which shows up on the IR spectrum as a broad peak at 3756cm-1
Original animation by Nick Greaves for ChemTube3D
Water has a bending vibration which shows up on the IR spectrum as a peak at 1595cm-1
Original animation by Nick Greaves for ChemTube3D
The next post runs through the detail of interpreting an IR spectrum for a range of molecules, with some example exam style questions and a focus on language and terminology ð.